Some are also pondering something else: Just how big is 7 billion really?

It's a number that's easy to underestimate.

On the surface, it doesn't look much different than 6 billion, either in written form or numeric form. There are nine zeros in 6,000,000,000, just like there are nine zeros in 7,000,000,000. But if you counted every number in between them, it would take more than 30 years. Yes, three decades.

And when was the last time you used 7 billion in everyday life? Have you ever eaten 7 billion of something? Have you ever owned 7 billion of anything? Early this year, there were only 140 people in the world who were worth $7 billion, according to Forbes magazine.

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"The number is just outside of our usual everyday scale of thinking," said Klaus Volpert, an associate professor of mathematics at Villanova University. "We count to 10 on our fingers and that's our scale, you know? Even counting to a million is already kind of outside of the everyday experience. And then once you go past a million, it becomes a blur."

Here are some different ways that might help you envision the enormousness of 7 billion:

-- Seven billion seconds ago, the year was 1789. That was the year George Washington was inaugurated as the first U.S. president and Congress met for the very first time.

-- If you took 7 billion steps along the Earth's equator -- at 2 feet per step -- you could walk around the world at least 106 times.

-- Suppose an average thimble holds 2 milliliters of water. Seven billion of those thimbles would fill at least five Olympic-sized swimming pools.

-- Let's say the average human is about 5 feet tall, accounting for children. If you stack those 7 billion people end to end, they would reach about 1/14th of the way to the sun -- or 27 times the distance to the moon, Volpert said.

-- Seven billion ants, at an average size of 3 milligrams each, would weigh at least 23 tons (46,297 pounds).

"Our mind just staggers," Volpert said, when thinking about how big 7 billion really is. He said there's a similar feeling when trying to grasp ultra-tiny measurements or something as vast as outer space.

"Every once in a while when we look up at the stars, we get a glimpse of the scale that's beyond the human scale, and it's fascinating and great," he said.

Population experts are hoping that more people begin to grasp the 7 billion concept soon, because the number has skyrocketed in recent years and the situation is becoming more urgent. (See populations country-by-country)

The world didn't reach 1 billion inhabitants until 1800, according to the Population Reference Bureau, and it reached 2 billion in 1930. But with advances in modern medicine, it took only 30 more years to reach 3 billion, 14 years to reach 4 billion, 13 years to reach 5 billion and 12 years to reach both 6 billion and 7 billion.

The U.N. has estimated a population of 9.3 billion by 2050, and there is expected to be more than 10 billion people on Earth by 2100.

"We're getting into more and more trouble the bigger the number gets," said John Bongaarts, vice president of the Population Council, an international nonprofit group. "Every billion people we add makes life more difficult for everybody that's already here."